Tuesday, 23 May 2017

How a beneficial gut microbe adapted to breast milk

Breast
milk provides vital nutrients not only to infants, but also to beneficial
microbes that inhabit the gastrointestinal tract. A new study shows that a
bacterial species called Bifidobacterium
longum has successfully adapted to the unique niche of the infant gut by
producing an enzyme called LnbX, which enables this microbe to grow on a sugar
that is abundant only in human milk.

Gut
microbes in early life are thought to have long-lasting effects on human
health, and studies have shown that diet strongly influences the composition of
this population. For example, human milk sugars are known to selectively
promote the growth of beneficial gut microbes such as Bifidobacteria, which prevent diarrhea and pathogenic infection in
infants. One major component of human milk is a sugar called lacto-N-tetraose,
which is virtually absent in the milk of other mammals. Bifidobacteria produce enzymes that break down this sugar, strongly
suggesting that a symbiotic relationship recently evolved between these
microorganisms and humans.

While
investigating how this symbiotic relationship evolved, Katayama and co-senior
study author Shinya Fushinobu of the University of Tokyo previously
characterized LnbB and isolated LnbX -- enzymes that degrade lacto-N-tetraose
in Bifidobacterium bifidum and Bifidobacterium longum, respectively. In
the new study, the researchers set out to build on these findings by
determining the X-ray crystal structure of the catalytic domain of LnbX. The
crystal structure, in combination with mutation and pharmacological
experiments, revealed that LnbX has a distinct structure and catalytic
mechanism from LnbB and therefore belongs to a novel family of glycoside
hydrolase enzymes called GH136.